Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are ordinarily arranged into a membrane electrode assembly (MEA). An external circuit conductor electrically connects the electrodes to a load, such as an electronic circuit. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OH−) in alkaline electrolytes. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized as the fuel at the anode of the fuel cell. Similarly, the oxidant can be any material that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density. At the fuel cell cathode, the most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications.
When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are preferably porous to permit the gas-electrolyte junction to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the metallic external circuit. At the cathode, oxygen gas reacts with hydrogen ion migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically extracted through evaporation. The overall reaction that takes place in the fuel cell is a sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. As long as hydrogen and oxygen are fed to a properly functioning fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte.
Fuel cell electrodes have been made by a variety of processes but usually involve the mixing of carbon material, catalyst material, and electron conductive material, and the disposition of this mixture onto membrane support structure. One such process is described in U.S. Pat. No. 6,280,872 issued to Ozaki et al. on Aug. 28, 2001 and entitled, “ELECTRODE FOR FUEL CELL AND A METHOD FOR PRODUCTING THE ELECTRODE”. Here, an electrolyte membrane is coated with catalyst material and a paste containing carbon particles overlaid onto the catalyst-coated membrane. Another process is described in U.S. Pat. No. 5,738,905, issued to Bevers on Apr. 14, 1998, and entitled “PROCESS FOR THE PRODUCTION OF A COMPOSITE COMPRISING ELECTRODE MATERIAL, CATALYST MATERIAL, AND A SOLID-ELECTROLYTE MEMBRANE”. Here, a catalytic powder consisting of electrode material, catalyst material and solid-electrolyte material is heated to form a composite which is used for an electrode.
The performance of a fuel cell depends in part on the number of reaction sites present in the fuel cell electrodes. For example, a cathode must have available three-phase boundary reaction sites for efficient operation. One problem often found with prior art fuel cells is that the number of reaction sites is limited as a result of the process used in manufacturing the electrode. FIG. 1 shows a fragmentary cross-sectional view of a typical prior art fuel cell electrode 100. In construction, ionic conductor material or ionomer 102, catalyst material 103, and electron conductor material 104 are mixed in a paste or slurry and the electrodes formed from this mixture on a membrane structure 105. As a result, the electrode has a random structure. For the reaction to occur at the cathode, the catalyst material, electron conductor material, ionic conductor material, and oxygen must be simultaneously in contact with each other. As the structure is a random mixture of material, many of the catalyst sites are placed such that they are buried and inactive. Additionally, the ionic conductor must be permeable to oxygen. As in many prior art implementations, the ionomer coats the carbon and catalyst particles to varying degrees of thickness. Oxygen diffuses slowly through areas having a thick coating of ionomer, thereby limiting fuel cell performance. Particularly, a reduction in oxygen diffusion rate generally translates to a reduction in maximum current in a fuel cell. In certain cases, some of the catalyst sites have little or no ionomer coating. When the electrode is used as an anode, the absence an ionomer coating means that the reaction rate at that catalyst site is limited, as there is no proton path to the catalyst site. This limits the low current performance of the fuel cell.
An improved electrode is needed to address the electrochemical issues described for prior art fuel cell. The electrode structure should also be improved to provide for increased management control for water produced at the reaction sites.